The invention relates to a method for controlling the air-fuel mixture in an Otto engine having at least one cylinder, especially in an automobile engine, for the optimum use of a catalyzer with a probe which serves to determine the air ratio in a hot exhaust gas stream with carbon monoxide, nitrogen oxides and hydrocarbons, in which the measuring signals of the probe are converted with an electronic means into control signals for the ignition and mixture preparation. The invention further relates to a device for carrying out the method.
An Otto engine, an engine with a spark ignition, e.g. an automobile petrol engine, a stationary petrol engine or a two-stroke petrol engine, produces essentially carbon dioxide, but also carbon monoxide, nitrogen oxide and hydrocarbons, which are discharged into the environment. Whereas the carbon monoxide and the nitrogen oxides relate to chemically defined compounds, the hydrocarbons comprise a large number of compounds.
Automobile engines especially these days are subjected to increasingly strict exhaust gas standards which permit only limited residual quantities of the harmful gases mentioned. In the exhaust gas cleaning technology for Otto engines driven by petrol, a central role is played by catalyzers controlled by a lambda probe, especially three-way catalyzers, but also oxidation and/or twin-bed catalyzers. In a series of chemical reactions, for example in a three-way catalyzer, the harmful exhaust gas components, especially also the heterogenic hydrocarbons, are converted with approximately 90% efficiency into water, carbon dioxide, nitrogen and hydrogen.
The simultaneous reaction of the harmful exhaust gases of carbon monoxide, nitrogen oxides and hydrocarbons assumes that a ratio of the air-fuel mixture around the stoichiometric value .lambda.=1 is adhered to within strict limits. Lambda is also used to represent the air ratio: ##EQU1## .lambda.&lt;1 refers to a rich mixture, .lambda.&gt;1 refers to a lean mixture.
A lambda probe supplies a voltage signal to an electronic control which in turn signals to a mixture preparation device whether the mixture is to be enriched or made leaner. However, the change in the air-fuel mixture must not take place suddenly, an integrator changes the mixture composition and, if necessary, it changes the ignition timing slowly as a function of time. The air-fuel mixture thus deviates constantly by a few percent around .lambda.=1, even during optimum operation.
According to the present-day state of the art, lambda probes are arranged before the catalyzer in the exhaust gas flow direction approximately 1 m down-stream behind the exhaust gas quarter bend, in the collecting pipe in the case of multi-cylinder engines.
A lambda probe operates according to the principle of a galvanic oxygen concentration cell with a solid body electrolyte of zirconium dioxide which is stabilised with yttrium oxide. The solid body electrolyte is protected with a metal mesh--also known as metal cage--, which dampens the force of the hot exhaust gas stream. The exhaust gas flows round the outer surface of the solid body electrolyte, while the inner open chamber is connected to the atmosphere as reference gas.
Despite its obvious advantages, the lambda probe also has disadvantages:
It operates only at temperatures from approximately 400.degree. C. upwards. During the critical period prior to the catalyzer being heated, the air-fuel mixture is not regulated, or the lambda probe must be pre-heated.
The maximum continuous operating temperature is approximately 800.degree. C. The probe must be arranged in a cooler area of the collecting pipe, relatively far back in the flow direction.
The lamda probe has a response time of 50 to 100 msec. which, together with the long delay of the exhaust gas prior to reaching the probe, amounts to a very long reaction time for engines operating at high speeds.
The lambda probe provided with a metal mesh or metal cage has a large effective cross-section and therefore substantially interferes with the exhaust gas stream.
The CH-A5 666 724 describes a lambda probe which has a solid body electrolyte withdrawn from the exhaust gas stream. Whilst the lambda probe seated on a tubular holder is less likely to be overheated and does not interfere with the exhaust gas stream, being situated further back, however, it is even more sluggish with regard to the reaction time, the exhaust gas stream flowing unhindered no longer flows directly round the solid body electrolyte, but only after a branched-off partial stream has passed through a relatively small opening.
The DE-C1 3 743 295 sets out to prevent the probe ceramics from being destroyed by liquid droplets falling onto a heated lambda probe during cold starting. Also, the representation of the signals delivered by the lambda probe is intended to be improved. For this purpose a gas deflection plate is arranged upstream of the lambda probe in such a way that the lambda probe is situated in the flow shadow. The lambda probe protected by a relatively large cage of known construction is arranged in the collecting pipe of the exhaust system and captures part of the entire exhaust gas stream of all the cylinders in a region with an already relatively low gas temperature.
It is the object of the present invention to provide a method of the type mentioned at the beginning and a device for carrying out the method which permit a more selective engine control with probes and associated control electronics which respond substantially more quickly than was hitherto the case.